The present invention relates generally to a method and apparatus for controlling sea borne seismic data acquisition systems comprising an array of streamers, and in particular to the relative vertical and horizontal positioning of seismic streamers forming a seismic array towed behind a sea borne towing vessel.
The related art discloses a wide variety of towed marine seismic tracking and positioning systems and methods typically comprising one or more seismic streamers and/or one or more seismic sources. Some these seismic tracking and positioning systems and methods utilize a main or host vessel and/or other-associated unmanned vessels or vehicles to tow the seismic array. Typically towing is controlled or guided by a central control system. The known marine seismic tow tracking and positioning systems and methods are comprised of apparatuses such as seismic hydrophone streamers and attached floats, paravanes, and/or buoyant members. Typically each streamer and control apparatus is connected to the host vessel by a line, cable or tether. Considerable towing power is required of a host vessel to tow existing seismic streamer systems, cables and interconnecting sensing devices. A typical host vessel is capable of towing a plurality of tow vessels and/or carrying a plurality of seismic streamers or arrays and associated support apparatus on the vessel""s deck awaiting deployment.
One such tracking and positioning system for positioning and control of marine seismic streamers is taught in the international application published under the Patent Cooperation Treaty (PCT), International Publication Number WO 00/20895, international publication date Apr. 13, 2000. A marine seismic system with independently power tow vehicles is taught by U.S. Pat. No. 6,028,817. A control device for controlling the position of a marine seismic streamer is taught in the international application published under the Patent Cooperation Treaty (PCT), International Publication Number WO 98/28636, international publication date Jul. 2, 1998.
With known seismic cable tracking and positioning systems, the location and spacing of system components is limited by the type, size, and length of cables used and by the characteristics of the towing vehicles and other control devices utilized by the known seismic tracking and positioning systems. Typically, the plurality of towed seismic streamers form an array which is towed behind the host vessel. Typically, changing the configuration of such a towed streamer array, comprised of known components is a complex, cumbersome, time-consuming operation and can often become somewhat unwieldy. Moreover, the spatial and temporal positioning capability of known towing and tracking and positioning systems is limited.
There is no known seismic tracking and positioning system that enables independent and relative positioning of individual seismic streamer array elements, for example xe2x80x9cbirdsxe2x80x9d, seismic streamers, comprising sensors, sources and depth and position controls, for example to configure, manipulate and/or maintain a desired geometry of and within a towed seismic streamer array. There is also no known seismic tracking and positioning system that enables relative positioning and manipulation of an entire seismic streamer array. There is also no known tracking and positioning system that enables specification of a plurality of diverse acquisition and ancillary non-acquisition array geometries that facilitates run-time maintenance, retrieval and deployment of a towed seismic array. Moreover, there is no known seismic tracking and positioning system that tracks the geometry of the seismic streamer array and the relative positions of the individual streamers comprising array elements with respect to time and with respect to the earth""s latitude and longitude so that towed seismic array data acquisition runs are repeatable, thereby enabling acquisition of four-dimensional geophysical data (x, y, z, time).
Thus, there is a need for a seismic acquisition tracking and positioning system that overcomes the above-mentioned shortcomings of known seismic data acquisition tracking and positioning systems. There has been a long-felt need for an efficient and effective towed marine seismic tracking and positioning system having system components that are easily and reliably tracked, controlled and positioned. There has also been a long-felt need for seismic data acquisition towing, tracking and positioning systems to provide sufficient positioning flexibility to enable efficient, accurate, and repeatable control of the relative and absolute horizontal and vertical positions of towed arrays and the streamers, sensors and sources within a towed array during seismic data acquisition.
The present position invention provides a method and apparatus for an active tracking and positioning system for a towed seismic streamer array. The present invention recognizes and addresses the previously mentioned shortcomings, problems and long-felt needs associated with known towed seismic tracking and positioning systems. The present invention provides a solution to the aforementioned problems and provides satisfactory meeting of those needs in its various embodiments and equivalents thereof.
The active tracking and positioning system of the present invention provides a method and apparatus that enables relative positional control of any number of towed seismic streamers. The present invention controls streamer positions horizontally and vertically using active control units positioned on each streamer within the seismic array. The three component (x, y, z) position of each streamer element, relative to the vessel, relative to each other and relative Earth coordinate latitude and longitude is controlled, tracked and stored with respect to time during each seismic data acquisition run. This stored data is referred to as legacy data. Environmental factors (wind speed, currents, temperature, salinity, etc), and maneuverability data for the streamers and geometry of the towed array (cable diameter, array type, deployed configuration, vessel type, device type, etc.) for the seismic data acquisition run are also sensed with respect to time and stored as legacy data. The acquisition of legacy data enables repetition of seismic data acquisition runs.
The present invention provides active streamer positioning devices (ASPDs) as control elements, which are installed on individual streamers comprising an array, alternatively at the diverter position, streamer head, along the length of the streamer or at the tail of the streamer. One or more ASPD is employed on each seismic array. The active control elements are also installed at the head tow point of an array, at the head or tail of one or more streamers comprising an array, and/or along any streamer comprising an element of an array. The control elements attached at the streamer head are alternatively detachably connected to the front of the array to facilitate detachment and removal of a single streamer element from the array during a data acquisition run.
The apparatus and method of the present invention enables a seismic array to be maneuvered as the towing vessel maintains it desired course or as the towing vessel maneuvers to assist in the repositioning of the array during a seismic data acquisition run. The present invention may alternatively employ methods such as force vectoring, active wings, or other known means of changing the spatial or temporal, that is the vertical or z-depth position of the streamer elements comprising the array. In addition to enabling change in the spatial and temporal positions of individual array elements, the tracking and positioning system enables maintenance of specific array position and geometry in the presence of variable environmental factors. In this mode, the control system of the present invention adjusts the lifting force of the streamer to accommodate variations in the tide, real-time current velocity and direction, water depth, towed speed, cable maneuverability, towed direction and water salinity, and salinity variations, all of which may affect the buoyancy and position of the towed seismic array. The salinity of the water in which the array is towed may vary abruptly when a salt water towing operation passes by the mouth of a fresh water supply such as a river. The salinity may diminish considerably in the fresh water region near the mouth of a river, thereby requiring adjustment of the array towing forces to maintain desired array geometry and depth.
The present invention enables repeatable, four-dimensional seismic data acquisition by sensing and storing environmental data, temperature sensitive array maneuverability data and positional tracking data for a towed array and each individual element of the towed array with respect to time. The relative horizontal and vertical positioning of each seismic cable and energy source independent of the streamer attached to the array or streamer is controlled, monitored and stored, along with real-time environmental data during a seismic data acquisition operation. The composite stored data comprises legacy data for the data acquisition run. Thus, a seismic data acquisition run can be duplicated at a later date to reproduce the same array geometry and path during subsequent data acquisition runs, in the presence of varying environmental and maneuverability conditions. That is, a particular seismic data acquisition operation is repeatable based on the legacy data, thereby enabling repeatable four-dimensional data (x, y, z, time) gathering for multiple runs over a seismic target area. The present invention also enables implementation of non-acquisition ancillary array configurations such as augmented array geometry for collapsing or expanding an array during deployment or retrieval. The present invention also enables raising a single array element for retrieval from a deployed array during acquisition.
One array section or one streamer of an array section can be deflected out of the array geometry during a data acquisition run and vertically to the surface for detachment, retrieval and/or maintenance. The present invention also enables compensation for a failed or missing streamer. Streamers adjacent a failed or missing/removed streamer can be moved closer together to compensate for the failed or missing streamer array element with or without removal of the element. This enables repair and/or removal and replacement of such a failed array element during a seismic data acquisition run without interruption of the data acquisition run to repair and or replace the disabled streamer. Runtime removal of streamers and compensation for failed streamers enables continuation of data acquisition without causing the vessel and/or array to deviate from its planned data acquisition path.
It is time consuming, difficult and expensive to interrupt the towing vessel during a data acquisition run. Interruption for replacement or repair a streamer element in a data acquisition array and a subsequent attempt to duplicate array geometry and reposition the array and necessitates starting the vessel and array again at the exact location and array geometry where the run data acquisition run was interrupted. This process is difficult and time consuming, if not impossible. The present invention enables compensation for an array element and continuation of a data acquisition run during repair and replacement of the array streamer element.
The instant invention also enables configuring the towed array geometry to compensate for changing environmental and operational conditions that affect maneuverability. The array geometry can also be configured to increase or decrease the temporal resolution and spacing of the array to avoid ghost notching. Deployment and ancillary configurations are also provided wherein, for example, the width of an array may be increased to reduce chances of streamer tangling during deployment. The present invention also enhances in fill shooting where an optimal path has been selected. The present invention enables movement of the array in conjunction with movement of the vessel along an optimal in fill shooting path. The present invention individually controls positioning of the vessel and positioning of the array. Thus, the present invention also enables the towing vessel to account for a portion of the positioning along the selected optimal in fill path or data acquisition path and positioning of the array to account for the remainder of the positioning along the path. This division of positioning movements between the vessel and the array provides a more flexible and efficient positioning system for use during in fill shooting and/or other data acquisition positioning maneuvers.
In one aspect of the invention a seismic streamer array tracking and positioning system is presented comprising a towing vessel for towing a seismic array; an array comprising a plurality of seismic streamers; an active streamer positioning device (ASPD) attached to at least one seismic streamer for positioning the seismic streamer relative to other seismic streamers within the array; and a master controller for issuing positioning commands to each ASPD for maintaining a specified array geometry. In another aspect of the invention the apparatus further comprises an environmental sensor for sensing environmental factors which influence the path of the towed array. In another aspect of the invention the apparatus further comprises a tracking system for tracking the streamer positions versus time during a seismic data acquisition run and storing the positions versus time in a legacy database for repeating the positions versus time in a subsequent data acquisition; and an array geometry tracking system for tracking the array geometry versus time during a seismic data acquisition run and storing the array geometry versus time in a legacy database for repeating the array geometry versus time in a subsequent data acquisition run.
In another aspect of the invention the apparatus further comprises a master controller which compares the positions of the streamers versus time and the array geometry versus time to a desired streamer position and array geometry versus time and issues positioning commands to the ASPDs to maintain the desired streamer position and array geometry versus time. In another aspect of the invention the apparatus further comprises a master controller that factors in environmental factors into the positioning commands to compensate for environmental influences on the positioning of the streamers and the array geometry.
In another aspect of the invention the apparatus further comprises a master controller which compensates for maneuverability in the positioning commands to compensate for maneuverability influences on the positioning of the streamers and the array geometry. In another aspect of the invention the apparatus further comprises a monitor for determining the status of each streamer, wherein the master controller adjusts the array geometry to compensate for a failed streamer. In another aspect of the invention the apparatus further comprises a monitor for determining the status of each streamer, wherein the master controller adjusts the array geometry to move a failed streamer out of the array. In another aspect of the invention the apparatus further comprises a monitor for determining the status of each streamer, wherein the master controller detaches a failed streamer from the array. In another aspect of the invention the apparatus further comprises an array geometry which comprises a plurality of streamers positioned at a uniform depth. In another aspect of the invention the apparatus further comprises array geometry comprising a plurality of streamers positioned at a plurality of depths for varying temporal resolution of the array.
In another aspect of the invention the apparatus further comprises an array geometry comprising a plurality of streamers positioned along a plane, wherein the plane is rotated at an angel theta with respect to the longitudinal axis of the array. In another aspect of the invention the apparatus further comprises an apparatus wherein the array geometry is tracked via satellite and communicated to the master controller.
In another aspect of the invention a method is provided for tracking and positioning a seismic streamer array comprising a towing vessel for towing a seismic array; providing a seismic streamer array comprising a plurality of seismic streamers; providing an active streamer positioning device (ASPD) attached to each seismic streamer for positioning each seismic streamer; providing a master controller for issuing positioning commands to each ASPD and to the towing vessel for maintaining an optimal path, wherein the master controller further comprises a processor for calculating an optimal path for the seismic array for optimal coverage during seismic data acquisition over a seismic field, and a streamer behavior prediction processor which predicts array behavior, wherein the master controller compensates for predicted streamer behavior in issuing positioning commands to the towing vessel and the ASPDs for positioning the array along the optimal path, wherein the master controller compensates for environmental and maneuverability factors in the positioning commands.